Fair Value Measurements - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Fair Value Measurements
Transfer of assets from Level 1 to Level 2 of the fair value measurement hierarchy	$ 0	$ 0
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Money market funds
Fair Value Measurements
Investments in money market funds	108,585,000	97,402,000
Recurring basis | Quoted prices in active markets for identical assets (Level 1) | Corporate debt securities
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis | Significant other observable inputs (Level 2) | Money market funds
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis | Significant other observable inputs (Level 2) | Corporate debt securities
Fair Value Measurements
Investments in money market funds	13,875,000	29,425,000
Recurring basis | Significant unobservable inputs (Level 3) | Money market funds
Fair Value Measurements
Investments in money market funds	0	0
Recurring basis | Significant unobservable inputs (Level 3) | Corporate debt securities
Fair Value Measurements
Investments in money market funds	$ 0	$ 0